There are organic compounds that have been known to exhibit superconductivity at normal pressures. Examples of such organic compounds include bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) compounds, as described in JP-A-61-277691, dimethyl(ethylenedithio)diselenadithiafulvalene (DMET) compounds, as described in JP-A-63-246383, and methylenedithiotetrathiafulvalene (MDT-TTF) compounds reported by G. C. Papauassiliou et al. in the International Conference on Science and Technology of Synthetic Metals, Jun. 26-Jul. 2, 1988, Santa Fe. (The term "JP-A" as used herein means an unexamined published Japanese patent application.)
It is also known that cationic radical salts such as (BEDT-TTF).sub.2 Cu(NCS).sub.2 and the deuteride thereof (BEDT-TTFd.sub.8).sub.2 Cu(NCS).sub.2 have critical temperatures (Tc) of 10K and more, as described in H. Urayama et al., Chem. Lett., p. 55 (1988).
These organic superconductors are generally produced by an electrolytic crystal growth method. According to this method, an electron donor such as BEDT-TTF and an electron acceptor such as I.sub.3.sup.- or Cu(NCS).sub.2.sup.- or raw materials therefor are dissolved in a suitable solvent to form an electrolytic solution and, using a Pt electrode and the like, a direct current is permitted to flow through the electrolytic solution so that the electron donor and the electron acceptor undergo an electrolytic redox reaction to produce an organic superconductor as a crystalline charge transfer complex.
However, the electrolytic crystal growth method has a problem in that an extremely long time is needed to produce an organic superconductor although it depends on the reaction temperature, current value and other conditions. For example, in the case of producing an organic superconductor from a solution of 30 mg of an electron donor in 180 ml of a solvent by performing electrolytic crystal growth at a reaction temperature of 20.degree. C. with a current at 1 .mu.A, it takes from several days to several weeks to produce 5 to 10 mg of the desired crystal, as described in JP-A-2-270224.
As a further problem, the yield of the product based on the starting materials is very low, and this fact, combined with the long time required for completing the process, makes the electrolytic crystal growth method unsuitable for use in large-scale production of organic superconductors.
H. Mu/ ller, one of the inventors of the present invention, has made an attempt at oxidizing BEDT-TTF with Cu(NCS).sub.2 in a solution to synthesize (BEDT-TTF).sub.2 Cu(NCS).sub.2 in a short time and successfully produced the intended superconductor having Tc of 8.8K by stirring an acetonitrile solution of BEDT-TTF and Cu(NCS).sub.2 for 1 hour with heating at the boiling point of the solvent acetonitrile (as described in Synthetic Metals, vol. 39, pages 261-267 (1990)). However, this method requires heating and, in addition, the solubility of the starting materials in the solvent is low and the yield of the reaction product has not been necessarily satisfactory.
The present invention has been accomplished under these circumstances.